The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of fabrication and verification processes for ICs, and, for improvements to be fully realized, further developments in IC manufacturing are needed.
As merely one example, advances in lithography have been important to reducing device size. In general, lithography is the transfer of a pattern from a mask onto a workpiece such as a semiconductor substrate. In photolithography, the radiation used to transfer the pattern causes changes in affected areas of a photosensitive material formed on the workpiece. After exposure, the photosensitive material can be selectively removed to reveal the pattern. The workpiece then undergoes processing steps that take advantage of the shape of the remaining photoresist to create features on the workpiece.
In one type of lithography, extreme ultraviolet (EUV) radiation (i.e., radiation having a wavelength of about 1-100 nm) is used to transfer the pattern. However, because few materials are transmissive to EUV radiation, a complex system of reflective optics is used to direct and shape the EUV radiation. Accordingly, in many EUV systems, the mask containing the pattern to be transferred to the substrate is also reflective. EUV lithography poses a number of challenges, many of which arise from the arrangement of the reflective optics. For example, many reflective masks are not perfectly planar, and the thickness of the absorber formed on the mask may be much larger than the wavelength of the EUV radiation. Therefore, if the incident radiation is not completely perpendicular to the surface of the reflective mask, undesirable shadows and other 3D effects may occur. For these reasons and others, despite remarkable advances in EUV lithography, further improvements have the potential to deliver improved resolution, improved alignment, and improved yield. Therefore, while existing lithographic techniques have been generally adequate, they have not proved entirely satisfactory in all respects.